Superconductivity is a phenomenon occurring in certain materials at extremely low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field (known as the Meissner effect). An electric current flowing in a uniformly continuous loop of a superconducting wire, segment or cable can persist indefinitely with no power source. However, uniformity of the electric current is limited by the length of the superconducting wire, segment or cable. In order to increase a length of a superconducting wire, segment or cable, superconducting wire segments are connected to each other through joints.
U.S. Pat. No. 7,226,894 to Raber et al. discloses cleaning two endings of superconducting cables by metallic non-superconducting sheath to expose superconducting surfaces. Subsequently, the superconducting surfaces are pressed against each other to form a bridging connection. Spot welding is also disclosed for forming the connection. The exposed surfaces can also be heated to obtain continuity reaction between them.
World Intellectual Property Organization Document No. WO 2007/128635 A1 discloses arranging superconducting wires in a sleeve or bush, adding magnesium diboride (MgB2) and reducing the sleeve.
U.S. Pat. No. 4,713,878 to Kumpitsch et al. discloses putting superconducting wires in a solidified superconducting mold metal.
U.S. Pat. No. 5,134,040 to Benz et al. discloses connecting wires with metals constituting niobium tin (Nb3Sn) superconductor followed by heating the connected wires in order to obtain reaction between metals and thus creating a superconductor bridging.
U.S. Pat. No. 5,255,837 to Xu et al. discloses a method of forming a low resistance lap joint suitable for joining magnet coils in a superconducting magnet assembly on a magnet drum by forming a flat solder sandwich within a groove on the drum and moving a heat source along the lap joint with controlled temperature, pressure and rotational speed of the drum.
U.S. Pat. No. 5,604,473 to Rodenbush and U.S. Pat. No. 5,581,220 to Rodenbush et al. disclose short bridging segments of superconducting material that are used to electrically connect individual double pancake coils together in a series circuit, where the bridging segments are made of the same material used for winding the coils themselves.
These known methods suffer from a number of drawbacks which makes them unsuitable for creating joints between superconducting segments that achieve superconductivity using a granular superconducting material, such as magnesium diboride (MgB2), which can not be easily pressed or reduced. In one arrangement, U.S. Pat. No. 7,337,527 to Grasso et al., which is hereby incorporated by reference in its entirety, discloses a method of manufacturing superconducting wires having a plurality of filaments comprising a magnesium diboride (MgB2) granular superconducting substance. The application of high pressure to such filaments can cause breakage. On the other hand, the application of lower pressure to such filaments would produce inefficient connections between superconducting filaments for carrying a useful amount of current.
Further, the use of so-called “low temperature superconductors” (for example, niobium tin (Nb3Sn), niobium titanium (NbTi) and niobium aluminide (Nb3Al), etc.) and the related techniques for joining them produce a “low temperature superconducting joint.” This low temperature joint structure, however, eliminates the high critical temperature advantage of magnesium diboride (MgB2).
Meanwhile, the use of the so-called “high temperature superconductors” (for example, Bismuth strontium calcium copper oxide (BSCCO), Yttrium barium copper oxide (YBCO), etc.) and related technique for joining them produces a “high temperature superconducting joint.” Due to flux pinning instabilities, a typical problem with this type of high-temperature structure is that only a very-low resistance joint is created and it is not possible to obtain a real superconducting joint.
Therefore, a general need exists for efficiently joining superconducting segments that comprise granular superconducting substances.